Packed instructions are carried out on packed operands. A packed operand comprises a data string consisting of a plurality of sub-strings, each defining a particular data value and referred to herein as "objects". Thus, an operand comprises a plurality of objects. Each operand is stored in a register store which has a predetermined bit capacity addressable by a single address and in which individual bit locations cannot be individually addressed. Thus, it is not possible to address and handle individual objects within each operand.
"Packed instructions" allow a common operation to be carried out on all of the objects within an operand without individually identifying them. A simple common operation is to load the operand into and out of memory.
Another "packed instruction" is an arithmetic instruction which performs the same arithmetic operation on pairs of objects from respective operands in parallel to provide a packed result operand comprising a plurality of result objects. For the execution of packed arithmetic instructions, a computer provides a packed arithmetic unit which operates on two source operands, at least one of which is packed, to generate a packed result.
It is clearly advantageous to deal with a set of objects in a single operand together, because it reduces loading and storing operations to memory and maximises the use of available register capacity by filling each register.
However, in a packed arithmetic environment, separate steps must be taken to deal with individual objects within an operand. That is, to add together objects within a single operand, each object would have to be separately loaded into a register store before it could be combined with another object. This not only wastes register capacity but also requires memory accesses, which can be slow. It further requires a plurality of instructions, which increases the length of instruction sequences.
In an article entitled "UltraSpark adds Multimedia Instructions" in Microprocessor Report of 5th Dec. 1994, an instruction entitled PDIST exists. This instruction is a single instruction which acts on the contents of a register which holds eight data values resulting from a partition substraction of two 64 bit registers. The instruction adds together the absolute value of each of the eight results. Each result represents the difference of the objects within the 64 bit registers. The PDIST instruction then further adds the resulting sum to the accumulated difference value.
The PDIST instruction is therefore a highly specialised instruction for performing one particular cumulative operation.